An optical isolator is an optical component allowing the transmission of light in only one direction. It is typically used to prevent various parts of an optical system from reflection-induced disturbance.
Typically, an optical isolator comprises a magneto-optical element called a Faraday rotator which is sandwiched between a pair of polarization elements commonly referred to as a polarizer and an analyzer. The Faraday rotator is used in optical devices, such as the optical isolator, to rotate the plane of polarization that is incident upon it by a predetermined amount, usually by 45° either clockwise or counter clockwise. The magneto-optical crystal Terbium Gallium Garnet (TGG) is an optimum material for the Faraday rotator. TGG has a combination of excellent properties such as a large Verdet constant (defined as the polarization rotation angle per unit path length and per unit magnetic field strength), low light loss, high thermal conductance and high light damage threshold. An external magnetic field generated by a magnet (e.g. a permanent magnet) penetrating the magneto-optical element is required to activate the Faraday rotator. The direction of Faraday rotation is dependent on the orientation of the magnetic field but not on the direction of light propagation.
In the forward direction, light incident on the polarizer will pass through the polarizer without obstruction if its direction of polarization coincides with that of the polarizer. When this light passes through the Faraday rotator its direction of polarization is rotated by 45° due to the magneto-optic effect. The direction of rotation, that is, clockwise or counter clockwise, is dependent on the particular Faraday rotator configuration and is predetermined. The light is then transmitted through the analyzer without loss, since the direction of polarization of the analyzer is oriented at the same 45° relative to the polarizer.
In the reverse direction, back-reflected light of arbitrary polarization is incident on the analyzer which transmits some of this light and polarizes it to match its direction of polarization. When this polarized reflected light passes through the Faraday rotator its direction of polarization is again rotated by 45°, clockwise or counterclockwise relative to the direction of light travel, as is predetermined. As a result, the direction of polarization of the back-reflected light incident on the polarizer is perpendicular to its direction of polarization, and, thus the back-reflected light is blocked by the polarizer. In this manner, the optical isolator is used to transmit light from a source in the forward direction and essentially extinguish any reflected light in the reverse direction.
The magnitude of the rotation of the direction of polarization of light transmitted through the Faraday rotator depends on several factors, such as, the strength of the magnetic field, the nature of the material that constitutes the Faraday rotator, the wavelength of the light, the temperature, and other parameters. The components in many optical applications utilizing the Faraday effect are exposed to temperature variations. Hence, the temperature dependency of the Faraday rotator limits their use in devices which do not provide some form of temperature compensation to prevent or minimize degradation in performance. Since the isolation (the attenuation in the reverse direction) of an optical isolator is measured very close to zero, small temperature-induced changes can have orders of magnitude effects on the degree of isolation in terms of the transmission of back-reflected light in the reverse direction.
One solution to this problem proposed in the prior art is to provide temperature compensation via a cooling/heating source which maintains the temperature of the Faraday rotator. This requires that the temperature of the Faraday rotator be monitored and the output from the cooling/heating source be adjusted accordingly. Thus, the components required in such a temperature compensation system would include at least a cooling/heating source, temperature measurement device, a feedback system, and a power supply. This disadvantageously adds to the complexity and cost of the optical isolator.
A temperature-compensated optical isolator is known from U.S. Pat. No. 6,252,708 B1. In the approach disclosed in this document, the optical isolator utilizes a bimetallic element to rotate the polarizer or the analyzer in response to temperature variations. This self-actuated tuning achieves a blocking of the light in the reverse direction even if the Faraday rotation is different from 45° due to temperature variations. The optical isolator thus maintains an effectively constant isolation over a substantially wide temperature range. However, the drawback is that the insertion loss of the optical isolator becomes higher and higher the more the Faraday rotation deviates from 45°.
It is further known in the art that tuning of the Faraday rotator for temperature-compensation can be achieved by variation of the magnetic field that acts along the length of the magneto-optical element. Such variation can be effectuated by a shift (displacement) of the magneto-optical element (wherein the position of the magnet is kept fixed) or, vice versa, by a shift (displacement) of the magnet (wherein the magneto-optical element is kept fixed). DE 195 06 498 C1 discloses an optical isolator consisting of a circular cylindrical CdMnTe crystal as a Faraday rotator sandwiched between two polarizers within two identical NdFeB annular permanent magnets arranged in a common housing. A set screw driven into a threaded bore in the top of the housing has a ferromagnetic tip whose movement in the axial direction is followed by the magnets. The set screw is operated manually for the purpose of compensating for temperature-induced variations of the Faraday rotation angle. The drawback of this approach is that manual tuning of the optical isolator is required which inhibits applications in which significant temperature variations occur and manual interventions are not feasible for practical reasons.
Against this background it is readily appreciated that there is a need for an improved optical isolator with temperature-compensation that is simple, low cost and dimensionally small.